Currently, it is desirable to have small antennas with broadband and/or multi-band transmitting and receiving capabilities for telecommunications and other applications. In particular, such antenna structures are preferably miniaturized and in the shape of a thin disk or other similar planar structure for mounting, for example, on cellular telephones, microcomputers, vehicles, or other equipment.
One well-known and widely used antenna which, at least to some extent, meets the foregoing requirements and, consequently, has been deemed by many to be a possible candidate for such applications is a microstrip patch antenna. However, microstrip patch antennas generally suffer from narrow bandwidth and relatively large size as measured by the operating wavelength of such devices. Researchers have attempted to reduce the size of microstrip patch antennas while, at the same time, expanding their bandwidth with little success.
In addition, generally, electrically small antennas, which are defined to be antennas that can be enclosed in an electrically small volume measured by their operating wavelength, are inherently limited in their gain bandwidth. Such antennas invariably exhibit low directivity or a broad beamed radiation pattern such as an omni-directional antenna epitomized by a short dipole. Consequently, such antennas have a low gain since EQU [Antenna Gain]=[Efficiency].times.[Directivity]
where the efficiency of the antenna includes the effect of dissipative losses due to the lossy properties of practical conducting and dielectric materials of which such antennas are constructed, and the effect of losses due to any impedance mismatch with respect to the antenna feed line. The antenna efficiency is generally always less than 100% since the construction materials are inevitably lossy, and the impedance matching is virtually always imperfect, especially over a wide frequency bandwidth.
It is worth noting that in practice, an electrically small antenna is often said to have a low gain referring to the fact that it has a low efficiency. A relatively high efficiency is necessary when the antenna is employed to transmit a signal, or is employed in broadcasting and two-way telecommunications. For further review, these concepts are discussed in books such as K. Fujimoto, A. Henderson, K. Hirasawa, and J. R. James, Small Antennas, Research Studies Press, Letchworth, Hertfordshire, England, 1987; and K. Fujimoto and J. R. James, ed., Mobile Antenna Systems Handbook, Artech House, Boston, 1994.
Efforts to reduce the size of the antenna by slow-wave (SW) techniques have been very unsuccessful resulting only in marginal reduction in antenna size.
Due to the foregoing limitations, research to develop compact high-efficiency disk-shaped antennas for broadband and/or multiband operation has met with limited success. While other electronic devices have seen a dramatic reduction in size, most notably integrated circuits and the like, antenna size reduction has been an extremely difficult technological barrier to overcome. Further, this barrier is currently one of a relatively few technological barriers for wireless telecommunications and other wireless systems.